This invention resides in the technology of nozzle construction for supersonic propulsion systems.
Rocket-powered launch vehicles require high thrust at takeoff to overcome inertia, particularly since the vehicle is at its greatest weight due to the unburned fuel. Takeoff typically occurs at sea level, but at high altitude, where the vehicle performs its primary mission, a high specific impulse (Isp) is desired. This is most readily achieved when the nozzle has a high area ratio, which is the ratio of the area at the nozzle exit to the area at the throat. Achieving a high thrust at sea level and a high area ratio are conflicting goals, since at sea level the high area ratio results in a wall pressure near the nozzle exit that is below ambient pressure. This causes a negative thrust in the portion of the nozzle near the exit, lowering the overall thrust.
Certain attempts to eliminate the negative component of the sea level thrust while still achieving high vacuum thrust have involved the use of variable area nozzles, i.e., those in which the area at the exit is reduced for launch and then gradually increased during ascent. To achieve this, variable area nozzles have thus been designed to permit adjustments to the contour, area ratio and length as the vehicle altitude increases. Unfortunately, altitude compensation features such as these add considerable complexity as well as weight to the engine construction, and in most cases the nozzle still yields less thrust at sea level than at vacuum. Dual-fuel concepts have also been proposed. These include kerosene-fueled engines combined with engines derived from the Space Shuttle Main Engine (SSME), engines that combine kerosene-fueled engines combined with hydrogen-fueled engines such as the Russian RD-701 engine, the dual-fuel, dual-expander engine concept as set forth in Beichel, R., U.S. Pat. No. 4,220,001 (issued Sep. 2, 1980), and the dual-thrust rocket motor of Bornstein, L., U.S. Pat. Nos. 4,137,286 (issued Jan. 30, 1979) and 4,223,606 (issued Sep. 23, 1980). The Beichel engine requires a complex nozzle design that incorporates two thrust chambers, while the Bornstein motor achieves dual thrust by using separate sustainer and booster propellant grains in the combustion chamber, together with an igniter and squib that are inserted into the grain itself.
The present invention resides in a jet propulsion system that achieves both high thrust at sea level and high Isp at high altitude, without the thrust reduction at sea level that is otherwise caused by exit region wall pressures that are below ambient. To achieve this, the system includes a supersonic nozzle of continuous curvature in which secondary combustion occurs in an annular region of the interior of the divergent section of the nozzle. The secondary combustion forms a secondary combustion gas that complements the primary combustion gas passing through the nozzle. The secondary combustion gas maintains a wall pressure that is equal to or greater than ambient pressure at low altitudes, eliminating the negative component of the takeoff thrust. The invention is thus well suited to high area ratio nozzles, and particularly to nozzles that are otherwise overexpanded. Vehicles to which this invention can be applied include the Space Shuttle Main Engine (SSME), single-stage-to-orbit (SSTO) vehicles, and other vehicles in which the thrust at sea level might otherwise be compromised by the need for high Isp at high altitudes. The benefits of this invention can be achieved with only slight modifications to existing nozzles and with no change to the nozzle geometry.
These and other features, embodiments and advantages of the invention will be apparent from the description that follows.